For me I think its therapy as much as anything. I do stuff myself because I don't understand how I ended up with a white collar desk bound job when I'm much happier outside with dirty hands.
And then there is the ego component. I'd rather say I did it myself than I paid for it myself...
And I am cheap. I paid about $2,500 to have some work done on my Corvette a while back. When the work was done I was sick realizing how much of it was labor and that was money I could have put into tools for my shop. I could have done that work myself.
More recently I paid $4,000 for a transmission replacement in my wife's Subuaru. I could have done that one myself too, and seriously considered it, but not as much of it was labor, and since its the only car my wife will drive I would have needed to rent her a car while it was down. Safe and possibly cheaper answer was paying to have it done. And no one will ever ask who did the work on that stupid car.
But on projects like this truck, it just doesn't matter if it sets in the shop another year or two, and so I do it myself as time and money and ideas allow. Some weekends I spend more time and money on coffee and beer than the project, and that's okay with me.